This invention relates to a process for monitoring a continuous casting mold in operation. More particularly this invention relates to a monitoring process for measuring the level of liquid metal, the thickness of the slag layer, and the thermal conditions within the mold wall.
In the operation of continuous casting molds, it is important to know the level of liquid metal in the mold. Knowledge of level of metal in the mold makes it possible to adjust an appropriate refill or an appropriate billet drawoff rate. Reliable and rapid measurement methods must be used for measuring the level of liquid metal filling the mold. The appropriate measurement devices must also be usable with closed systems under powder or under inert gas; and the measurement devices should be installed so that they do not interfere with or obstruct the inlet to the mold; and at the same time they must be protected themselves from exposure to and damage from the metal. Furthermore, such measuring devices should represent no sources of risk.
Processes are known in which the presence of liquid metal is deduced from the temperature of the mold wall. However, such processes are sluggish and costly. Optical processes are also known. However, their usefulness is limited when the pouring is done under powder or in closed systems. Measurement methods are also known in which radioactive substances are used. However, these systems are encountering increasingly strict licensing procedures.
It is generally known that currents are induced by changes of the magnetic fields penetrating a conductor, and that the presence of the conductor can be deduced from the secondary fields of these currents. A measurement method employing this concept is described in German Patent Application Disclosure 2,101,729. It is also known that the impedance of a coil changes when a conductor is placed in the field of the coil. Thus, in accordance with French Pat. No. 2,251,811, a contactless induction instrument can determine the position of the boundary between two phases which have different densities and resistivities, with at least one of the phases conducting the current being nonmagnetic. In that French Patent, the measuring instrument includes operating and measuring coils which are oriented towards the phase boundary in open arrangement.
However, these prior art measurement methods fail in the continuous casting of steel, since the highly conductive copper mold used in such casting screens the fields so strongly that the sensitivity of the known measuring devices is unsatisfactory. Furthermore, measuring devices which are located above the mold or on the top edge of the mold impair accessibility to the mold and can be destroyed by overflowing steel.
Luxembourg Patent Application No. 80,410 (assigned to the assignee hereof and the disclosure of which is incorporated herein by reference) describes how the effect of the copper mold can be eliminated by a field-producing primary coil and two identical secondary coils wired in opposition, together with the liquid metal, forming a system in which the position of the metal level can produce an induced voltage in the coils. As taught in that Luxembourg patent application, the primary coil and the secondary coils lie coaxially around the vessel; the voltage induced in the secondary coils on the basis of the position of the metal level, as well as the electrical conductivity of the metal, are measured; and the position of the metal level is determined from the voltage corrected for the measured conductivity. However, experience with this measurement method has shown that, with large wall thickness of the mold or in the case of a thick, hot covering of the steel pool by molding powder or slag, the measurement obtained is not the steel level but rather the slag level or a weighted value of the two. This occurs because the conductivity of the copper mold changes with temperature and the temperature is modified by contact with hot slag or powder. With thick molds, the measured signals can be influenced more strongly by the temperature-induced conductivity changes in the mold wall than by the presence of the liquid metal.